Multiple h1 tags means that you have more than one h1 tag on the page. While a page can have multiple h2, h3, h4, or other heading tags, it’s important to only have one h1.
Why it is important
The headings used to organize content in the body of an HTML document have different hierarchical levels, starting with the least important (h6) and ending with the most important (h1). Each of these headings corresponds to a specific section of the document and is typically formatted in a style that allows distinguishing visually between headings of different levels. The h1 heading (h1 tag) is typically formatted with the largest font size.
Headings can be viewed as elements of a document’s outline. In this case, the h1 tag corresponds to the article’s title, h2 headings represent top-level chapters or content sections, and headings of lower levels are sub-chapters or sub-sections.
Having just one h1 heading per article is important because it makes it easier both for humans and search engines to identify the hierarchy of the content but also because the importance of the keywords wouldn’t be diminished due to excessive content in h1 tags.
Matt Cutts, the Former Head of the Webspam Team at Google
How to check the issue
Look into the source code of your HTML document with this simple tool at https://codebeautify.org/source-code-viewer.
Alternatively, use the in-built browser tools with similar functionality – in Google Chrome, one can do this by right-clicking on a blank region or text and choosing “View page source.”
In the source code, look for text flanked by <h1> – the latter designates h1 headings.
How to fix the issue
Make sure you leave only one h1 tag – the simplest way to do this might be to convert h1’s into h2’s.
Check your website for multiple h1 tags
Audit your website to detect whether pages have two and more h1 tags